1. Field of the Invention
The present invention relates to an optical element, namely reflector that alters the properties of light intentionally interacting with it. The present invention also relates to an optical element incorporated in to a display device and a method of manufacturing such a display device.
2. Description of the Related Art
Reflective display devices are well known. In principle, these consist of a light-modulating element, and a reflector disposed behind the light-modulating element. Light incident on the front of the light-modulating element passes through the element, is reflected by the reflector, and passes back through the light-modulating element. A reflective device had the advantage that, under suitable illumination conditions, it can utilise ambient light and does not require its own light source. For such a display to operate effectively, it is necessary that sufficient of the ambient light incident on the display is directed towards the observer so that a sufficiently bright display is produced.
Blazed reflectors can be used to redirect ambient light impinging on a reflective display at an oblique angle so that, after reflection, it exits the display substantially at normal incidence. This is advantageous since viewers of a display generally view the display from the normal direction, or from a near-normal direction, and the use of a blazed reflector creates a higher reflectance of the display towards such a viewer.
The principle of operation of a blazed reflector is illustrated in FIGS. 1(a) and 1(b).
In FIG. 1(a), light is incidence on a block 1 of material with a refractive index ni which has upper and lower surfaces that are parallel to one another. If light is incident on the front surface of the block 1 at an angle xcex80 to the normal, it will undergo refraction at the front surface of the block 1. It will propagate within the block at an angle xcex8i to the normal, where xcex80 and xcex8i are connected by Snell""s Law, namely sin xcex8i=sin xcex80/ni (assuming that the medium outside the block has a refractive index n0=1).
A reflective layer 1xe2x80x2, such as a metallic or dielectric layer, is disposed on the lower face of the block 1 of FIG. 1(a). Light transmitted through the block 1 is specularly reflected by the reflective layer 1xe2x80x2 when it reaches the lower surface of the block, and is again refracted at the upper surface of the block so that it leaves the block at an angle xcex80 to the normal. It can thus be seen that if light is incident on the upper surface of the block 1 at an oblique angle to the normal to the block, it will be reflected at an oblique angle of the same absolute value, and so will not reach an observer viewing the block from the normal direction.
The advantages of using a blazed reflector are illustrated in FIG. 1(b). In FIG. 1(b), the lower surface of the block is not a plane surface parallel to the upper surface of the block, but is in the form of a blazed reflector. As is well known, the reflective surface of a blazed reflector consists of segments, with each segment being inclined at an angle xcex8m (known as xe2x80x9cthe angle of blazexe2x80x9d). A reflective layer 1xe2x80x2 is disposed on the lower face of each segment.
Since the lower surface of the block is a series of inclined segments, light transmitted through the block at an oblique angle will be reflected closer to the normal of the display. Indeed, if the angle of blaze is chosen such that xcex8m=xcex8i/2, then the reflected light will be reflected in the normal direction. Use of a blazed reflector will thus increase the brightness of the display in a normal or near-normal direction.
If the refractive index of the block 1 is assumed to be ni=1.5, the angle of blaze required to direct reflected light in the normal direction is xcex8m=10xc2x0 for the case xcex8030xc2x0. If xcex80=45xc2x0, then the required angle of blaze is xcex8m=14xc2x0.
FIG. 2(a) is a polar diagram showing the preferred reflection cone 2 from a reflective display device for collimated light that is incident on the display at an azimuthal angle of 90xc2x0 and a polar angle of +30xc2x0. If light is reflected within this cone, it will reach an observer viewing the display at a normal, or near-normal, angle.
FIG. 2(b) is a polar diagram showing a typical range 3 of possible positions for a light source for use with a reflective display device that incorporates a blazed reflector.
Reflective display devices are known which consist of a conventional liquid crystal display device, and a blazed reflector disposed behind the liquid crystal display device. A blazed reflector suitable for this application is typically produced by embossing a thermoplastic polymer film 4 disposed on a substrate 5, by moving a suitable embossing tool over the layer of photopolymer. This is illustrated schematically in FIG. 3. Once the photopolymer layer 4 has been embossed, a metallic film is then disposed over the photopolymer 4 to produce the blazed reflector. Manufacture of a blazed reflector in this way to described in U.S. Pat. Nos. 5,245,454 and 5,128,787.
Simply disposing a blazed reflector behind a conventional passive matrix LCD is only satisfactory if the extension of the second substrate normal to the display plane is small compared to the extension of the picuture element (pixel) in the display plane. This approach is not compatible with active matrix LCDs, since components of the active matrix LCD, for example such as thin film transistors (TFTs), will shade the blazed reflector. A significant amount of light passing through the LCD will thus not reach the blazed reflector, but will instead be reflected at an oblique angle by the pixel electrodes or absorbed by other components of the LCD. Furthermore, the use of a blazed reflector that is external to the LCD can give rise to optical cross-talk between adjacent pixels, and it can also cause parallax problems. This leads to a loss of resolution of the display.
It is desirable for a blazed reflector to be disposed within a LCD, so that the problems of shading of the reflector and of optical cross-talk and parallax are eliminated or at least reduced in severity.
A further desired property of a reflector for use in a reflective display is that it reflects light into a range of angles around the exact direction of specular reflection. If such a reflector is used, it is then possible for the direct specular reflection of a light source to be directed away from the position of an observer while still ensuring that a significant amount of light is directed to the observer. This means that the glare at the observer""s position is reduced, as the observer will not see an image of the light source.
U.S. Pat. Nos. 5,204,765, 5,408,345 and 5,576,860 in the name of Sharp Kabushiki Kaisha describe an internal electrode for a liquid crystal display which has diffuse reflecting properties. The reflector will direct light into a range of angles around the direction of specular reflection, and it also will preserve polariation of light incident on the reflector. However, this reflector is a symmetric reflector, and this technology has not yet been extended to produce an asymmetric diffuse reflector. Indeed, no-one has yet produced a blazed reflective TFT electrode suitable for internal use in a high resolution active matrix display. Furthermore, no-one has yet produced an asymmetric reflector which has diffuse reflecting properties.
It is, in principle, possible to create an internal blazed reflector using the embossing technique illustrated in FIG. 3, but in practice it is difficult to do this. One particular problem that arises with an active matrix LCD in which the blazed reflector is acting as a pixel electrode is that the metallic layer disposed over the photopolymer must be electrically connected to a thin film transistor (TFT) disposed on the substrate of the LCD. This requires that a through hole, or via, is created through the photopolymer layer 4, but this is difficult to do using conventional embossing techniques.
P. F. Grey discloses, in xe2x80x9cOptica Actaxe2x80x9d, Vol. 25, pp. 765-75 (1978), forming optical diffusers having a defined profile using a laser speckle pattern. Coherent laser light is optically diffused, and is used to expose a photoresist. Different diffuser properties can be obtained from the photoresist, for example by varying the light dose, the exposure duration, or by using single or multiple exposures.
A known method for creating a grating structure in a layer of photoresist is a near field holography method which uses self-interference of the zero and first order diffraction below a phase shift mask. This is shown schematically in FIG. 4. Light from a light source 7 is directed onto a transmission diffraction grating 8. Zeroth order light transmitted by the grating 8 is directed by a first mirror 43 towards a phase shift mask 9, while first-order diffracted light from the grating is directed by a second mirror 44 towards the phase shift mask. The zeroth order light and the first order light interfere with one another, and a standing wave extending a few microns below the phase-shift mask 9 is set up. This allows a layer of photoresist 4 disposed on a substrate to be exposed in the non-contact mode preferred in production processes. The main drawbacks of this technique are the cost of the phase-shift mask, and the limited mask size which also limits the size of the grating structure that can be defined in the photoresist layer. Furthermore, this method cannot produce angles of blaze that are sufficiently small for the present application of a blazed reflector for a display device.
Another method for defining a diffraction grating in a layer of photoresist is disclosed by M. C. Hutley in xe2x80x9cDiffraction Gratingsxe2x80x9d, Academic Press, London, 1982, pp. 95-125. This method is schematically illustrated in FIG. 5. Light from a light source 10, such as an argon ion laser, is expanded by the combination of a stationery diffuser 11 and a rotating diffuser 11xe2x80x2 and is then focused by a lens 12 to produce an expanded beam. This is incident on a semi-transparent mirror 13 which partially reflects the beam, and partially transmits the beam. The reflected portion is incident on a first mirror 14, and the transmitted portion is incident on a second mirror 15. The mirrors 14, 15 are arranged such that the two portions of the beam are directed onto a layer 4 of photoresist disposed on a substrate 5. An interference pattern is set up by the two beams, and the photoresist layer 4 is exposed by the interference pattern. This method has the advantage that either symmetric or asymmetric gratings can be defined in the photoresist layer, but it is disadvantageous since complicated optical path adjustment is required in order to set up the interference pattern. Furthermore, an asymmetric grating can only be produced if the layer 4 of photoresist is disposed on a transparent substrate. A TFT substrate is not uniformly transparent and contains opaque components.
U.S. Pat. Nos. 4,935,334 and 5,111,240 describe a method of producing a photoresist mask. These patents are specifically directed to varying the wall profile of a hole in the photoresist layer. These patents disclose a method of producing holes having a tapered wall profile, by partially exposing the photoresist layer through a mask, moving the mass and the photoresist layer with respect to one another, and subsequently exposing the photoresist layer again. Once the photoresist mask has been made, it is used as a mark in further processing steps, for example in the production of VLSI circuits; once these processing steps have been completed the photoresist layer is completely removed.
W. Dxc3xa4chner et al. (Appl. Optics, Vol. 36, p. 4675 (1997) describe a method for producing blazed structures in photoresist. It comprises the exposure of the photoresist with electromagnetic radiation through a grey-scale mask and the manufacture of a grey-scale mask.
A first aspect of the present invention provides reflective optical element comprising a microscopically structured surface with a reflective layer thereon. The reflective element allows for oblique incident light to be redirected and scattered into pre-determined directions by means of irregular piece-wise linear blazed structures. The reflective optical element may be manufactured by a method taught below.
A blazed structure is described by two angles. One describes the Shallow inclination (xcex8m in FIG. 1b), the other the steep inclination (90xc2x0 in FIG. 1b) with respect to the normal to the crest of a blazed segment. The blazed segment can have an additional curvature to it, for example convex or concave or a multitude of both in varying portions. Multiple segments are stringed together in a piece-wise linear method so that the plurality of normals to each segment""s crest vary over a range of azimuthal angles with respect to the averaged of the normal to all crests. The relative probability of occurrence of a particular azimuthal angle can be pre-determined using for example a Gaussian distribution as the probability function of a pre-determined range of azimuthal angles.
Compared to the prior art this invention has the following advantages: At the same time, the angle of blaze, its profile (concave, convex or both) and the azimuthal distribution and its relative probability of the blazed structure can be pre-determined. This allows the precise prediction of the distribution of the scattered reflection.
A second aspect to the present invention provides an electro-optic display device comprising: first and second substrates; a layer of electro-optic material disposed between the first and the second substrate; and a reflector as defined above disposed behind the electro-optic material when the electro-optic display device is viewed. In this second aspect the reflector has only the function as described in the first aspect of the present invention. This aspect of the present invention is particularly useful where the extension of the second substrate normal to the display plane is small compared to the extension of the picture element (pixel) in the display plane.
A third aspect to the present invention provides an electro-optic display device comprising: first and second substrates; a layer of electro-optic material disposed between the first and the second substrate; and a reflector as defined above disposed between the electro-optic material and the second substrate, wherein the said reflector has electrically conductive properties and acts an electrode to the electro-optic material. This aspect of the present invention is particularly useful where the extension of the second substrate normal to the display plane is large compared to the extension of the picture element (pixel) in the display plane and therefore would cause parallax problems.
A method of manufacturing an optical element may be comprising the steps of:
(a) exposing a first part of a layer of photoresist;
(b) exposing a second part of the layer of photoresist to a different depth than the first part of the layer of photoresist; and
(c) developing the photoresist.
When the layer of photoresist is developed, the resultant layer of photoresist will have regions of different thickness. Although the steps used in the method of this invention are similar to those disclosed in U.S. Pat. Nos. 4,935,334 and 5,111,240, in the present invention the layer of developed photoresist is incorporated into the optical element. This is in contrast to the prior art, which relate only to the production of a mask for use in subsequent processing steps such as, for example blocking liquid or dry etchants or as a graded resist for an ion implantation process. Once these processing steps have been completed, the mask of photoresist is completely removed.
This method of manufacturing an optical element is suitable for manufacturing an internal optical element within a display device such as, for example, an LCD or other electro-optical display device.
The method may comprise the further step of (d) disposing a reflective layer over the layer of developed photoresist. This will provide a reflector with an inclined reflecting surface.
The duration of the second exposure step may be different to the duration of the first exposure step. Alternatively, the intensity of radiation used in the second exposure step may be different to the intensity of radiation used in the first exposure step. These are convenient methods of exposing the second part of the layer of photoresist to a different depth than the first part of the layer of photoresist.
The photoresist layer may be exposed through a mask, and the mask may be moved relative to the layer of photoresist between the two exposure steps, or the mask may be moved relative to the layer of photoresist continuously during steps (a) and (b). Alternatively, the light source may be moved relative to the layer of photoresist between the two exposure steps, or the light source may be moved relative to the layer of photoresist continuously during steps (a) and (b). These are convenient ways of ensuring that different parts of the layer of photoresiut are exposed in the two exposure steps.
The method may comprise the further step of (e) exposing a third part of the layer of photoresist through the entire depth of the layer of photoresist, with this step being carried out before the step of developing the photoresist. This will create a through hole, or via, through the layer of photoresint, and this can be used to allow electrical connection between components on opposite sides of the layers of photoresist. For example, if the layer of photoresist is disposed over an active matrix substrate, the via can be used to connect a thin film transistor (TFT) on the active matrix substrate to an electrode disposed over the layer of photoresist.
The reflective layer may be an electrically conductive layer. The reflective layer can then be used as a reflective electrode. Furthermore, if a via has been created in the layer of photoresist, the step of disposing the electrically conductive layer over the photoresist will result in the via being filled with the electrically conductive material, thus creating an electrical connection through the layer of photoresist. This allows the reflective electrode to be connected to an associated switching element disposed on the other side of the layer of photoresist.
The reflective layer may be a metallic layer.
The mask used in the exposure steps may comprise a plurality of transparent lights defined in an opaque background. (The word xe2x80x9ctransparentxe2x80x9d and xe2x80x9copaquexe2x80x9d as used herein mean that the mask is transparent or opaque to the wavelength of light used to expose the photoresist.) Use of such a mask in the exposure steps will lead to the formation of a plurality of regions of reduced depth in the layer of developed photoresist, so that a blazed reflector having a plurality of inclined reflective surfaces will be formed when the reflective layer is disposed over the layer of developed photoresist.
The transparent lights defined in the mask may be piece-wise linear and irregular. In this case the xe2x80x9ccrestsxe2x80x9d of the blazed grating will not form straight lines, but will be irregular lines. This will mean that light reflected from the reflector will also be diffused, so that light will be reflected in a range of angles around the direction of specular reflection. Thus, a an asymmetric reflector that also diffuses lightxe2x80x94that is, one that reflects light into a range of angles around the direction of specular reflectionxe2x80x94is formed.
The transparent lines may be substantially parallel to one another. Alternatively, the separation between adjacent transparent lines may have random variations. These will result in corresponding random variations in the separation between adjacent crests of the reflector; these will cause scattering of reflected light in different azimuthal angles, and so will increase the diffuse nature of the reflected light.
The method may comprise the further step of (f) exposing the layer of photoresist to light having an intensity that varies randomly over the area of the layer of photoresist, this step being carried out before step of developing the photoresist. The exposure dose is set so that it does not fully expose the photoresist, but only creates additional, small variations in the thickness of the photoresist layer after development. These small thickness variations further increase the scattering of reflected light.
Step (f) may comprise exposing the layer of photoresist to a laser speckle pattern. The size and intensity of the laser speckle pattern will determine the size of the thickness variations in the developed photoresist layer.
While the method described above is preferred, the aspects 1 to 3 of the invention may also be achieved by employing a gray-scale mask which may be prepared by the method described above. In particular for the second aspect of the invention others methods of manufacture are conceivable, for example the embossing technique described above.
The method taught above provides a method of manufacturing an electro-optic display device comprising the steps of:
i) manufacturing an active matrix substrate;
ii) disposing a layer of photoresist over the active matrix substrate; and
iii) exposing and developing the layer of photoresist by a method defined above.
This allows the manufacture of a display device incorporating an internal optical element. As an example, an active matrix display can be provided with an internal blazed reflector, with the reflector also acting as the pixel electrode. The reflector can easily be connected to TFTs on the active matrix substrate, through a via in the layer of photoresist. Since the blazed ref lector is disposed over the active matrix substrate it is not shaded by any of the components of the active matrix substrate, in contrast to the prior art case of an external reflector disposed behind the display. Disposing the reflector within the display also reduces both parallax and cross-talk between adjacent pixels.